A part of model broken whilst posing
First, the mesh of your belt is not closed, you should delete some vertices and use W > LoopTools > Bridge. Also, it's not parented to the good bones. You should join it to the rest of the body ctrlJ, select your belt in Edit mode, remove from any group in the Properties panel > Data > Vertex Groups and Remove From All Groups function, then in Weight Paint you assign it to the bones that you want i.e. same bones as this part of the body) with the Limit Selection to Visible option disabled (on the horizontal menu bar) so that it paint the vertices you can't see.